Zeolite ZSM-18, the subject matter of U.S. Pat. No. 3,950,496, is an unusual zeolite first discovered by Julius Ciric at Mobil Oil Corporation in 1975. The entirety of U.S. Pat. No. 3,950,496 is incorporated herein by reference. The framework of ZSM-18 has been assigned the structure designation MEI by the International Zeolite Association Structure Commission and is reported to crystallize within a narrow Si/Al range of between 5.3 and 5.8. Indicated below is the structural formula of the triquatenary ammonium molecule that was first used to prepare zeolite ZSM-18, as disclosed in U.S. Pat. No. 3,950,496:

ZSM-18 has a hexagonal crystal structure in which 12-ring pores (6.9 Å in diameter) run parallel to the c-axis and a network of 7-ring pores (3.2 Å×3.5 Å) runs perpendicular to these large pores. In addition, ZSM-18 is unusual in that it includes 3-rings. Thus far, ZSM-18 is the only known aluminosilicate zeolite to possess 3-rings, probably because the bond angles required for 3-rings introduces strain into a siliceous framework. Other zeolite-like structures and known zeolites, such as lovdarite (LOV), nabesite (NAB), OSB-1 (OSO), OSB-2 (OBW), VPI-9 (VNI), VPI-7 (VSV), RUB-17 (RSN), RUB-23, RUB-29, RUB-30, ITQ-33, ITQ-40 and ITQ-44, possess 3-rings, but each of these zeolite-like structures has a substantial incorporation of a framework heteroatom that limits the stability of the zeolite or zeolite-like structure.
In particular, with reference to the above list, LOV, NAB, OSO, and OBW each contain framework beryllium, an element that allows for the narrow T-O-T angles that are generally observed in the 3-rings of zeolites. However, not only are these zeolites unstable toward dehydration and calcination, but the beryllium also presents toxicity issues in their preparation and handling. Furthermore, VNI, VSV and RSN are zincosilicates in which zinc resides in each of the 3-ring subunits; and RUB-23, RUB-29 and RUB-30 are each lithosilicates in which lithium resides in the T sites of the 3-ring units. The remaining ITQ materials are all germanosilicates, which also allow narrow T-O-T angles.
For years ZSM-18 has been considered a canonical example of the templating effects in zeolite synthesis due to the specific selectivity of the triquat used for its synthesis. Kirk Schmitt et al. examined other possible candidates that could be used to template or “structure-direct” ZSM-18 (see Schmitt, Kirk et al., Zeolites 1994, 14, 635) by matching the pseudo-symmetry and dimensions of candidate molecules with those of the zeolite framework. One such candidate is reproduced below:

More recently, it has been disclosed that a zeolite designated UZM-22, which is isostructural to ZSM-18, could be produced by the Charge Density Mismatch (CDM) approach with choline as the organic directing agent in conjunction with lithium and/or strontium cations (see U.S. Pat. No. 7,744,850). In the CDM approach to zeolite synthesis, which is disclosed in U.S. Publication No. 2005/0095195, quaternary ammonium hydroxides are normally employed to solubilize the aluminosilicate species in a first step, while crystallization inducing agents such as alkali and alkaline earth metals and more highly charged organoammonium cations are introduced to the synthesis mixture in a separate step. This approach has been used to make zeolite UZM-4 (U.S. Pat. No. 6,419,895), a BPH-type aluminosilicate that is stable to calcination.
According to the present invention, it has now been found that ZSM-18 can be successfully synthesized with butamethonium cations as the organic directing agent in conjunction with at least one cation selected from lithium, tetramethylammonium (TMA), strontium and sodium. In addition, it has been found that ZSM-18 can be synthesized in the presence of a combination of N,N,N,-trimethyl-N-butylammonium and tetramethylammonium cations. Zeolite ZSM-18 prepared by methods of the present disclosure is stable to calcination, and in its calcined form, has a large determined N2 micropore volume of 0.34 cc/g.